Frugivore Abundance Research Articles

Stopover Habitats of Landbirds During Fall: Use of Edge-Dominated and Early-Successional Forests

Abstract Despite much interest in the conservation of landbirds during migratory stopover periods, relatively few studies have examined spatial and temporal variation in habitat use and identified important habitats for migrating landbirds in North America. We surveyed migrant landbirds in five habitats (mature forest interior, mature forest-agricultural edge, mature suburban forest, mid-successional pole-stage forest, and early successional shrub-saplingstage forest) in central Pennsylvania from late August to early October, 1997–1999. We used abundances of individual species and migrant guilds, species richness, and fruit availability to assess relative habitat quality for fall migrants and measured structural characteristics associated with migrant habitat use. Of 15 species that differed in abundance among habitats, species that breed in mature forest (n = 10) were typically broadly distributed among habitats during stopover, with highest abundance in edge-dominated forests (forest-agricultural edge and suburban forest) and lowest abundance in pole-stage forests. Mature-forest-breeding migrants also regularly used early successional forests, where as many individuals were recorded as in forest interior. Shrub-sapling-breeding species (n = 5) generally were more narrowly distributed among habitats and were most abundant in early successional and edge-dominated forests. We detected among-year differences in relative use of habitats by mature-forest-breeding species, which suggests that the relative quality of stopover habitats may vary from year to year. Fruit availability was highest in shrub-sapling and forest-agricultural edge habitats and was positively associated with abundance of primary frugivores in all three years, indicating that fruit may be driving habitat selection by that guild. Mature-forest-breeding migrants were positively associated with forests that had more understory vegetation and lower percentage of canopy cover (i.e. more tree-fall gaps), which suggests that migrants selected sites with greater vertical and horizontal habitat heterogeneity. Migrating shrub-sapling-breeding species were positively associated with small-diameter stems (0–2.5 cm) and negatively associated with percentage of canopy cover (i.e. characteristics of breeding habitats). Consistently high use of mature edge-dominated and early-successional forests by a wide diversity of landbird species during fall stopover indicates the potential importance of those habitats for migratory landbird conservation.

BRIDGING THE GAP: INFLUENCE OF SEED DEPOSITION ON SEEDLING RECRUITMENT IN A PRIMATE–TREE INTERACTION

Frugivores are thought to influence plant recruitment by creating initial seed deposition templates. However, post‐deposition processes (e.g., predation) may obscure these initial patterns. Few studies have examined successive life‐history stages of animal‐dispersed plants to evaluate how initial frugivore seed deposition influences early life stages of plant recruitment. We addressed this issue by quantifying seed removal and seedling recruitment of Monodora myristica (Annonaceae), a tropical, low‐fecundity, forest tree in Kibale National Park, Uganda. Fruit morphology suggests that this species is dispersed by only the largest arboreal frugivores. We determined which frugivores removed fruit during focal tree watches and quantified seed fate in experiments designed to mimic natural deposition. We estimated stage‐specific transition probabilities of survivorship to seedling establishment and determined expected seedling recruitment from each deposition condition. To evaluate spatial and temporal variation, these methods were conducted at two sites, 15 km apart, over two years. Finally, we compared frugivore abundance between sites to expected seedling and standing seedling, sapling, and pole abundances. We found that large‐bodied primates were critical for seed dispersal. They were the only frugivores that opened the hard‐husked fruits and were estimated to disperse >85% of mature seeds. Seeds placed away from parents had higher germination and establishment probabilities than those under parents, indicating that seed dispersal is advantageous. Single seeds away from parents (mimicking small‐bodied primate seed spitting) had the highest cumulative seedling recruitment probability. However, in three of four site‐by‐year combinations, 86–94% of estimated recruits came from seeds placed in clumps in dung (mimicking large‐bodied primates). High recruitment in the latter condition is due to the large number of seeds estimated to be deposited by large‐bodied primates. Despite this concordance, germination and establishment probabilities were highly variable between sites and years. Germination was a limiting step in recruitment, and high seed mortality from beetles and rodents indicates the importance of predation. The site with higher frugivorous primate abundance had higher standing seedling abundance, but lower expected seedling recruitment and lower sapling and pole abundances. Thus, even in a system where frugivores are critical for dispersal, spatial and temporal variation in post‐deposition processes reduces predictability of frugivore actions on seedling recruitment.Corresponding Editor: E. S. Menges

Matrix matters for seed dispersal – a comment to Jules & Shahani

In a recent JVS Forum paper, Jules & Shahani (2003) shed some light on the role of matrix quality for conditioning habitat fragmentation effects on plant regeneration, focusing on processes modulated by plant-animal interactions. The authors gathered published information on how matrix characteristics modify the outcomes of pollination, seed predation and herbivory in fragmented landscapes, by providing occasionally suitable habitat for fragment species, determining patch connection or even buffering negative border effects related to among-habitats structural contrast. However, they could not find any evidence of these matrix effects on seed dispersal. In this note we pay attention to several observational and experimental studies suggesting, directly or indirectly, that matrix actually matters for seed dispersal by animals. A first suggestion of how matrix quality can modulate plant-disperser interaction comes from the evidence that matrix affects the abundance of vertebrate frugivores, providing the first step for differential seed dispersal patterns. For example, Estrada and Coates-Estrada (2002) showed that bats, many of them potential dispersers for a broad spectrum of plant species, were less abundant in tropical forest fragments surrounded by pastures, compared to patches embedded in an agricultural, structurally more complex matrix, namely cocoa and coffee plantations. Another observational, functional evidence is related with the role of remnant trees isolated in matrix pastures surrounding many tropical forest fragments (e.g. Galindo-Gonzalez, Guevara, & Sosa 2000). Remnant trees are used as perches and foraging canopies by frugivorous birds and bats, facilitating the movement of dispersers among fragments. Thus, they foster seed dispersal from forest into pastures, acting as nucleation foci for tree recolonization, and they promote seed exchange among forest fragments, functioning as ‘stepping stones’ for dispersal processes. Finally, a recent experimental contribution by Tewksbury et al. (2002) provides unequivocal evidence of the positive effect of habitat corridors for seed dispersal. Working in Matrix matters for seed dispersal – a comment to Jules and Shahani

Fruit biomass and relative abundance of frugivores in a rain forest of southern Western Ghats, India

Fruit biomass and frugivore abundance were quantified over 3 y in a rain forest of the south Western Ghats, India. Fruit biomass was estimated by sampling fruit fall in the primary forest, and frugivore abundance by a 2.5-km transect. A total of 645 kg ha−1 of fruit was produced annually in the forest. Only 49% of this is edible to the frugivores and the remaining 51% is in the form of non-edible husks. Mammalian frugivores outnumbered avian frugivores and the majority of the mammals were seed predators. The total fruit biomass produced at Kakachi is lower than in the lowland forest and mountain forests in the neotropics but higher than in the wet sclerophyll forest of Australia. Lower diversity of trees and edaphic factors at Kakachi could be some of the reasons for these differences. On the other hand, paucity of fleshy fruits, low density of trees producing fleshy fruits and irregular fruiting of these species, account for the low number of obligate avian frugivores at Kakachi.

ANTHROPOGENIC EDGES, TREEFALL GAPS, AND FRUIT–FRUGIVORE INTERACTIONS IN A NEOTROPICAL MONTANE FOREST

In a montane tropical forest in southwestern Colombia, we investigated how anthropogenic edges may alter bird-mediated seed dispersal from edge to forest interior as a function of edge age and presence of treefall gaps. We estimated fruit abundance and mist-netted birds at four distances from edge to forest interior (0–10, 30–40, 60–70, and 190–200 m) in three young ( 40 yr) edges. Fruit-sampling plots (50-m2 plots) at each of the four distances were classified into gap and intact forest. Fruit abundance and frugivore capture rates varied from edge to forest interior, but such changes depended on edge age. At new edges, the total number of fruits was higher at the forest edge than at the forest interior, whereas bird captures showed the opposite trend. At old edges, the total number of fruits and bird capture rates did not vary among the four distances. In a first group of 12 plant and four bird species, the distribution of individuals in fruit (7 species) and captures (3 species) from edge to forest interior differed between old and new edges. In a second group of 18 plant and five bird species, which included those that were not amenable for a comparison between old and new edges and those that were not influenced by edge age, the distribution of individuals in fruit (12 species) and captures (3 species) was not uniform from forest edge to forest interior. Lastly, 124 plant and 19 bird species with <20 individuals in fruit and captures, respectively, were classified into very sparse and sparse species. We found that all but the sparse frugivores were more abundant at the forest edge than in the forest interior. Because very sparse and sparse plant species showed such a clear trend, we used seeds retrieved from mist-netted birds to assess potential seed movement of these species from edge to forest interior. Seeds of very sparse and sparse plant species were found both at forest “edge” (0–10 m) and at forest “interior” (the three other distances combined). Our results suggest that birds are not responding to changes in fruit abundance (resource-base-driven mechanism). Instead, they indicate that frugivore capture rates reflect either a direct edge effect or a non-edge induced effect on birds. The apparent uncoupling of processes generating the observed patterns in fruit and frugivore abundance may affect seed dispersal in important ways. Furthermore, our results indicate that, as edges age, “edge effects” (i.e., maximum distance at which changes induced by edge creation are apparent within forest stands) change.

The phenology of growth and reproduction in plants

The study of phenological aspects of plants involves the observation, recording and interpretation of the timing of their life history events. This review considers the phenology of leafing, flowering and fruit production in a range of species and communities. The selective forces (both abiotic and biotic) that influence the timing of these events are discussed. Within the limits imposed by phylogenetic constraints, the phenological patterns (timing, frequency, duration, degree of synchrony, etc.) of each phase are probably the result of a compromise between a variety of selective pressures, such as seasonal climatic changes, resource availability, and the presence of pollinators, predators and seed dispersers. Many studies on flowering times stress the role of interactions between plant species which share pollinators or predators. The timing of fruiting plays a key role in controlling the abundance and variety of obligate frugivores in many tropical communities. The importance of long-term recording is stressed, particularly in species which fruit irregularly. An understanding of the phenology of plants is crucial to the understanding of community function and diversity.

Bird Community Dynamics along Primary Successional Gradients of an Amazonian Whitewater River

We used spot-map methods to census birds in two plots (40 ha and 80 ha) representing the early and middle stages of primary succession generated by the meandering of the Manu River in southeastern Peru in 1983 and 1985. We distinguished seven distinct successional stages beginning with the first plant communities growing on open beaches (Tessaria: stage 1) and proceeding through mature floodplain forest (stage 7). Each successional stage was more structurally and floristically complex. One plot was dominated by early successional vegetation (stages 1-5) and the other was domi- nated by middle-to-late stages (5-6). We supplemented spot-map data with mist netting to census nonterritorial species, principally nectarivores and frugivores. Because the ear- liest successional stage (Tessaria) covered only small (<3 ha) areas on any given me- ander tongue, we censused six additional stands to characterize their bird communities. Species richness increased along the successional gradient, but not uniformly. Struc- turally simple Tessaria stands (stage 1) contained more diverse communities of breeding birds than the next two successional stages, which appeared to have few resources avail- able to birds. After stage 3, breeding species richness increased by 31-71 species per successional stage as a distinct canopy layer formed (stage 4), fruiting trees became more available (stages 5-6) and the vertical structure of the forest became fully developed (stage 7: 71 more species than stage 6). Early successional bird communities (stages 1- 3) were dominated by a few very abundant insectivores and omnivores, but wanderers from the adjacent forest plus Nearctic and austral migrants seasonally outnumbered breeding residents. Many species that breed in Tessaria stands were restricted to just one or two of the seven stands censused. The middle stages of succession had the highest estimated richness, abundance, and biomasses of nectarivores and frugivores. Nectarivore abundance reflected the huge stands of Heliconia that dominate the understory of mid- successional stages. Bark foragers reached their highest abundance in late successional stages as the dominant canopy tree species of middle successional stages (e.g., Cedrela odorata) began to die off, but remained standing. Middle successional stages lacked many of the insectivores characteristic of more structurally complex mature forest. Among the missing species were understory Mockers, ant-followers, ground foragers, and many arboreal insectivores that participate in canopy flocks. Species of early and middle stages tended to be more abundant on average than those of mature forest. Early-/and late-successional bird communities had few species in common, and many congeners segregated along this successional gradient. Most species characteristic of early successional vegetation, however, were also found in many other kinds of natural and anthropogenic disturbances. The Tessaria beaches of the Manu lacked most species endemic to the same successional stage on the larger rivers in the Amazon system. Mid-successional stages had relatively fewer species restricted to them, but some of these species are rare and endemic to southwestern Amazonia. Many char- acteristic species of floodplains reached their peak abundances in stages 4-6. Mid-suc- cessional stages, therefore, may play a greater role in contributing to the unusually high species richness of western Amazonia than had previously been recognized. These results point to the importance of preserving natural, undammed meandering rivers in conserving the avian diversity of Amazonia. Logging and agricultural devel- opment of mid-successional floodplain forests probably has a negative impact on regional bird communities. Not all species of the early and middle stages of succession have adapted to human-generated secondary succession. Recovery of the full diversity and richness of tropical bird communities following disturbance may take several hundred

Temporal Variation in Birds and Fruits Along an Elevational Gradient in Costa Rica

We studied temporal fluctuations in fruit production by plants and in populations of understory fruit—eating birds at three elevations (50, 500, and 1000 m) in Costa Rican wet forests over a 12—16 mo period to investigate effects of resource variation on bird movements and community structure. We used mist nets to monitor changes in frugivore abundance, migration patterns, and breeding and molting cycles. We sampled understory fruits of each forest concurrent with studies of frugivores. Both frugivores and fruit exhibited considerable seasonal variation in abundance. Peak frugivore capture rates occurred during peak periods of ripe fruit abundance. Altitudinal migrants left lower montane (1000 m) forest during periods of fruit scarcity and wee present in lowland (50 m) and foothill (500 m) forest when ripe fruit was abundant. Migrants, both altitudinal and temperate, accumulated fat before migration, and perhaps (for altitudinal migrants) in anticipation of breeding. Some residents also put on fat before breeding. Breeding was seasonal at all forests and occurred when ripe fruit abundance was low. Results of this study indicate that birds may track changes in resource abundance. Thus, variation in resource abundance influences dynamics of bird communities, both in terms of species composition and abundance. Further, results illustrate the importance of viewing communities from different scales; dynamics at a local scale (e.g., one elevation) can be influenced by changes in conditions (e.g., fruit abundance) elsewhere. That some species regularly moved along elevational gradients implies that preservation of many species and of the biotic integrity of entire systems may require conservation of large, connected blocks of suitable habitat.

Spatial and Temporal Variation in Costa Rican Fruit and Fruit‐Eating Bird Abundance

Understory fruit and fruit—eating birds were censused monthly for a year in gaps, intact forest, and second—growth sites of a lowland Costa Rican rain forest. Both fruits and birds displayed significant seasonal variation. Peak fruit abundance corresponded with peak fruit—eating bird abundance. Fruits were most abundant in the mid—to—late rainy season (August—January). Crop sizes were larger on second—growth plants than on either gap or intact forest plants. Also, fruit was much more common in second growth than in gaps and more common in gaps than in intact forest. Fruit—eating birds followed the same general patterns of spatial and temporal variation. They were significantly most abundant in second growth, significantly least abundant in intact forest, and most common from October to January. A large increase in the frugivore population in October was due to an influx of temperate and altitudinal migrants. In addition, populations of some resident frugivore species increased concurrently, suggesting altitudinal migration in some of these species as well. The two most common understory frugivores molted during the period of fruit high and bred during the period of fruit low. I suggest that insect abundance may be more important than fruit abundance in determining breeding cycles of these birds. Given the extent of spatial and temporal variation in fruit and frugivore abundance, and the apparent tracking of fruit resources by birds, large areas of diverse habitats are probably necessary for the long—term maintenance of frugivore populations.

Patch Utilization by Migrating Birds: Resource Oriented?

Use of gap (created by tree falls) and non-gap forest understory sites by migrating birds in central Illinois was studied during spring and autumn for three years (19781980). Fruit and understory foliage were concentrated in gaps. Birds that relied on these resources (foliage-gleaning insectivores, frugivores in autumn) used gaps more than non-gaps. Birds that fed on food other than fruit and foliage insects (frugivores in spring, other insectivores) did not use gaps more than non-gaps. Bird abundance varied markedly among gap and non-gap sites, potentially reflecting differences in site preferences. Site selection, as determined by bird abundances, was consistent (correlated) between years for birds that fed on items that were concentrated in gaps but not for birds that did not rely on these patchy resources. Foliage density is a measure of foraging substrates for foliage-gleaning birds to search. Abundance of foliage-gleaning insectivores was highly correlated with foliage density in both spring and autumn. Frugivore abundance was highly correlated with fruiting foliage density during autumn when they are frugivorous, but not during spring when they are insectivorous. Insectivores not relying on foliage insects or fruit were uncorrelated with either index of resource availability. These same relationships hold even when examining gap sites only. Thus, migrants can be consistent in their selection of foraging sites and this consistency appears to exist when resource densities are markedly different among sites (patchy) but not when resources are more dispersed.

Fruiting and Fitness in Crataeugs Monogyna: The Effects of Frugivores and Seed Predators

The factors influencing variation in the rates of loss of fruit from Crataegus monogyna individuals are examined for one population near Liverpool, England. In contrast to the expectations from optimal foraging theory, there appears to be no simple relationship between fruit abundance and the activities of frugivores. A number of factors may influence rates of fruit loss including: bush width; inter-bush distance; the damage caused by seed predators. Because of correlation between fruiting characters there is a weak relationship between numbers of fruit and their consumption by frugivores. Fruit on larger, more fecund bushes also suffer high rates of seed predation. The outcome of opposing selective influences may be that fruit on smaller bushes have higher fitness. Fluctuations in frugivore and predator abundance may help maintain the observed high variance in fruit characters.

Phenology and ecology of color in bird-dispersed fruits, or why some fruits are red when they are "green"

Several hypotheses regarding the ecology of color in bird-dispersed fruits in North America are presented and tested. (1) We predicted, but failed to substantiate, an inverse relationship between size and colorfulness of fruiting display. Large fruit crops are more often bicolored than small ones. Red monochrome fruits are relatively common in herbs, but we cannot say if red is more "conspicuous" to birds than blue or black nor if herbs are relatively less conspicuous, when in fruit, than shrubs or trees. (2) Conspicuous displays are most common in plants that fruit at times when avian frugivore abundance is low. This hypothesis is supported strongly by the phenologies of temporally bicolored displays, which are produced commonly in summer before the influx of fall migrants. Data from morphologically bicolored displays are less clear-cut. Temporally bicolored displays may increase fruit dispersal particularly by juvenile birds and (or) decrease conspicuousness to invertebrate predators, as well as increase the level of display attractiveness to whatever dispersal agents are present at the time. (3) Conspicuous displays occur in regions where frugivore abundance is seasonally low. This hypothesis receives some support. A comparison of Illinois (low diversity and numbers of wintering frugivores) with the Carolinas (where both are high) suggested that monochrome displays may be relatively more common, and morphologically bicolored displays less so, in the Carolinas. High frugivore abundance and diversity may relieve competition for dispersors and reduce selection for unusually conspicuous displays.

Comparison of Avian Communities between Wet and Semiarid Habitats of Eastern Australia

The composition of bird species in three distinct habitats of subtropical eastern Australia was examined in terms of the degree of species association, niche occupation and clutch size. In spite of low species diversity in the wet formations, the birds of this habitat tended to show characteristics of tropical forest birds. These included stronger association of species, relative abundance of treenesting frugivores and small clutch size. In contrast, the semiarid formations supported a greater diversity of bird species with somewhat loose species association, a large representation of graminivores, and a larger mean clutch size with greater variation. At least in the subtropical region of eastern Australia, the bird species diversity does not appear to be related to the stability of environment.